1) Field of the Invention
This invention relates to a method of recording an information signal on an optical information recording medium by irradiating high density energy flux thereon.
2) Description of the Prior Art
Technology that an information signal is recorded and reproduced by using a laser beam is well known and there is an increasingly active trend recently to use it for document and data filing applications. In addition, examples of the research and development on the rewritable recording systems having a erasing function have been reported. Out of them, a phase difference type optical disk is one method available in which the state change either between an amorphous state and a crystalline state or between two different crystalline states is used. Thin recording film to be used in this method features the fact that either an amorphous state or a crystalline state is formed in accordance with temperature rising and cooling conditions when a laser beam is irradiated and that these states are reversibly changeable.
The amorphous state and the crystalline state are different from each other in the complex index cf refraction, which can be expressed by refractive index n and extinction coefficient k, and the signal recording is achieved using the thus generated difference in transmittance and reflectance of a recording medium. In order to realize this, when referred to, for example, SPIE's 30th Annual International Technical Symposium on Optical and Engineering, Conference 695: Optical Mass Data Storage 2,695-17, August, 1986, a method has been proposed in which a laser beam modulated between two power levels of writing power Pw and erasing power Pe (Pw&gt;Pe) is irradiated on a recording medium. When a laser beam with its strength modulated according to a signal to be recorded is radiated on the recording medium, a portion where the writing power Pw was irradiated becomes an amorphous state and a portion where the erasing power Pe was irradiated becomes a crystalline state no matter whether the state of an area to be irradiated thereon had been the amorphous or crystalline state before irradiation. As a result, writing and erasing are simultaneously carried out with a single laser beam, which means that an overwrite is carried out.
In the above, we described the change in state between the amorphous state and crystalline state, but when the change in state is between two different crystalline states, the same result as above can be obtained by corresponding the above-mentioned amorphous state to a second crystalline state.
As described above, writing on an optical disk is possible by irradiating a laser beam which has been modulated between two power levels. However, if a signal is reproduced after the overwrite has been repeatedly carried out, a component of the previous signal is superimposed upon the read signal due to incomplete erasures. Such superimposed signal would cause an error when the read signal is demodulated thereafter.
In addition, through detailed observation of the shape of a recording mark formed by the above-mentioned method, it was found that the mark thus formed is not symmetrical in shape at the beginning point and finishing point of writing and the read signal has a waveform distortion corresponding to the asymmetrical shape of the mark.
For this reason, when a recording method that one to closely determine both the rise position and the fall position of a reproduced signal waveform, such as, for example, the pulse width modulation (PWM) method, is to be introduced in order to improve the recording density of an optical disk, the above-mentioned conventional recording method cannot be used.